The Carbon cycle and reducing the Carbon footprint of agriculture
The Carbon cycle and reducing the Carbon footprint of agriculture Ram Dalal1,2, Diane Allen1 and Weijin Wang1 1 2
Queensland Department of Natural Resources and Water School of Land, Crop and Food Sciences, University of Queensland, St Lucia
Introduction Agriculture has always been about farming carbon. Until the beginning of industrial revolution, carbon outputs were in close synchrony with carbon inputs (plant biomass including vegetation fallows, manures, etc), and relatively small land area used for growing food and fibre crops. Carbon dioxide levels in the atmosphere before 1750 have been maintained between 180 ppm and 280ppm for at least 10,000 years and 380ppm, CH4 from 715ppb to 1774ppb and N2O from 270ppb to 320ppb by 2005 (IPCC 2007). Agriculture contributes about 16%, 40% and 36% of both natural and anthropogenic global emissions of CO2, CH4 and N2O emissions, so-called the carbon footprint of agriculture. We present the carbon footprint of Australian agriculture and management options to increase C sequestration and reduce N2O and CH4 emissions from agriculture. The Carbon cycle and major Greenhouse Gases (CO2, CH4 and N2O) Natural processes such as photosynthesis, weathering by silicate rocks and carbonate formation, and carbon fixed by marine plants and consequent burial of marine sediments provide the major carbon sinks. Respiration, decay, acidification of carbonates and sea-surface gas exchange are the major natural processes of carbon source. Fossil fuel burning, deforestation or land clearing followed by biomass burning are the major human-induced sources of CO2 into the atmosphere. CH4 is produced primarily by methanogens (Archea domain) by the process of methanogenesis under anaerobic conditions. On the other hand, CH4 oxidisers (methanotrophs) and consumers (sinks) are aerobic microbes in soil, requiring O2 for CH4 oxidation. N2O is produced in soil by at least three microbial-mediated mechanisms: (i) nitrification, utilising nitrite as an alternative electron acceptor, thereby, reducing it to N2O (via NO), (ii) dissimilatory nitrate reduction (denitrification) and (iii) assimilatory nitrate reduction. Over the last 250 years, coinciding with the beginning of industrial revolution, CO2 levels in the atmosphere have increased from 280 ppm to 384 ppm in 2008, the highest atmospheric CO2 concentration since at least 650, 000 years ago. Concentration of CH4 in the atmosphere has increased from 715 ppb to 1775 ppb and that of N2O from 270 ppb to 320 ppb since 1750AD. IPCC (2007) summarised the global greenhouse gas fluxes of C, CH4 and N2O (also O3) for 2005, which is presented in Figure 1. It has generally been accepted that CO2 emissions and sinks (as CO2 fixed during photosynthesis by vegetation and sequestered as soil organic C and carbonate C) in natural ecosystems vary very little. In fact, natural ecosystems provide net C uptake of -2.6 Gt (1 Gt = 1015 g = billion tonne) C/year, resulting in the additional terrestrial CO2 sink of ~ -0.9 Gt C/year (net release from land use change, 1.6 Gt C/year), as compared to C sink of -2.2 Gt C/year provided by oceans (Fig. 1). Thus, the natural ecosystems absorb almost 45% of anthropogenic GHG emissions of >7.2 Gt C/year, primarily from fossil fuel use and cement production. Almost two-third of CH4 emissions are from anthropogenic activities such as energy production and use, enteric fermentation and animal wastes, paddy rice, domestic sewage and landfills, and biomass burning. Almost 6% of atmospheric CH4 is consumed by bacteria in soil (Dalal et al. 2007). Emissions of N2O from human activities comprise 45% of the total global N2O emissions; of that 80% originates from soil. For total C stocks, refer to the IPCC (2007) report. In summary, surface oceans and surface sediments comprise 1050 Gt C, intermediate and deep oceans contain 37,100 Gt C, mostly in carbonate-C. Among the terrestrial systems, soil provides the major C sink (1550 Gt as organic C and 950 Gt as carbonateC), and vegetation provides 760 Gt C sink. Atmosphere contains about 760 Gt C, similar to that contained in vegetation, although 165 Gt C of the total stock is the annual C flux between atmosphere and land and oceans. In summary, the net GHG annual rate of increases in 2005 were estimated to be 4.1± 0.1 Gt of CO2-C (annual emissions, 650 Gt C, annual sinks, 646 Gt C), 2 Mt of CH4 (annual emissions, 582 Mt CH4, annual sinks, 580 Mt CH4), and 6 Mt of N2O (annual emissions, 28 Mt N2O; annual sinks, 22 Mt N2O).
Figure 1. Global GHG flux in 2005. (a) C fluxes showing an annual increase of 4.1 Gt C/year. Heterotrophic respiration (119.6 Gt C/year) and gross terrestrial primary production (~120 Gt C/year) are considered to have neutral effect on global C flux (not shown). (b) CH4 fluxes are dominated by anthropogenic emissions. CH4 oxidation in natural soils forms a significant CH4 sink. (c) N2O emissions from anthropogenic sources comprise almost 45% of the total annual N2O fluxes to the atmosphere. (d) Annual O3 fluxes; O3 lifetime in the atmosphere is less than a month. Global warming potentials of CH4 and N2O are 25 and 298 as compared to CO2 on 100-year time scale (IPCC 2007). The Australian greenhouse gas (GHG) fluxes Greenhouse gas flux estimates for Australia are provided annually (3-year moving averages) by the National Greenhouse Gas Inventory Committee, which is submitted to UNFCCC every year. The latest GHG estimates are available for 2006 (Table 1). Compared to 1990 baseline, GHG emissions in 2006 increased by 4.2% in line with 8% increase allowed for under Kyoto Protocol for the 2008-2012 period. The increase in GHG emissions from the energy sector by 40% (in case of Queensland by 90%) were substantially countered by decrease in GHG emissions from the land use change sector by 54% (about similar percent in Queensland) to enable to achieve the agreed Kyoto Protocol target. Table 1. National greenhouse gas emissions by sector in Australia for 2006 (Queensland values for the major sectors are given in parentheses) (NGGI 2008) Major sector
1990
Industrial processes Agriculture Waste Land use change Forestry
2006 Mt CO2-e /year 286.4 (50.6) 400.9 (96.0) 195.1 287.4 62.1 79.1 29.2 34.5 24.1 28.4 86.8 (23.6) 90.1 (26.4) 18.8 16.6 136.5 (89.0) 62.9 (41.7) 0 -23.0
Change (%) 40.0 (89.7) 47.3 27.4 18.1 17.7 3.8 (11.9) -11.4 -53.9 (-52.6)
Total net emissions
552.6 (169.8)
4.2 (0.7)
Energy Stationary energy Transport Fugitive emissions
2
576.0 (170.9)
Agriculture sector Total GHG emissions from agriculture sector were an estimated 90.1 Mt CO2-e emissions or 16% of net national emissions in 2006 (Table 2). Methane emissions from livestock and N2O emissions from agricultural soils, mainly from N fertiliser application were the dominant sources, accounting for 58.0% and 80.7%, respectively, of the net national emissions for these two gases. GHG emissions from livestock, including the enteric fermentation and manure management, declined by 4.7% (3.1 Mt) between 1990 and 2006, due to decrease in sheep numbers by 47%, which was partly offset by 14% increase in cattle numbers. There was a 30.6% (6.4 Mt) increase in emissions from the remaining agricultural activities between 1990 and 2006, mainly from increasing N fertiliser use and burning of savannas. However, there has been a small increase (3.8%) in overall GHG emissions from the agriculture sector between 1990 and 2006.
Table 2. Agriculture sector greenhouse gas emissions in 2006 (NGGI 2008) (emissions from enteric fermentation and agric. soils for Qld in parentheses) Category
CO2
Enteric fermentation
na
Manure management Rice cultivation Agricultural soils
na na na
Prescribed burning of savannas Field burning of agric. residues Total
na na na
Solid waste disposal on land
CH4 N 2O Mt CO2-e/year 59.3 na (21.9) 2.0 1.6 0.3 na na 15.2 (4.5) 8.1 3.4 0.2 0.1 69.8 20.3 13.2
0.6
Total 59.3
Category (%) 65.8
3.6 0.3 15.2
4.0 0.3 16.9
11.5 0.3 90.1 (26.4) 13.2
12.8 0.3 100
Total GHG (%) 10.3 (12.8) 0.6 0.05 2.6 (2.6) 2.0 0.1 15.6 (15.5) 2.3
In summary, there are three major categories of GHG emissions from the agriculture sector: CH4 emissions from livestock, N2O emissions from N fertiliser use, and CH4 and N2O emissions from savanna burning. For Queensland, the first two sources dominate GHG emissions from agriculture.
cotton sugarcane
Horticulture / vegetables
Irrigated crop Non- irrigated pasture
Irrigated pasture Non- irrigated crop
Figure 2. Direct GHG emissions from cropping soils and biomass burning were 27 Mt CO2-e in 2006, primarily from fertiliser N use and N mineralisation (NGGI 2008) Although sugarcane uses only 10% of total annual N fertiliser applied to cropping soils compared to 70% used for cereal crops, net GHG emissions are similar or even higher than those from cereal crops (Fig.
3
2). Currently, IPCC default value of 1.25% of fertiliser N for sugarcane is used in NGGI (Table 3). Field estimates of N2O emission factor varies from 1% (non-acid sulphate soils) to 10% (acid sulphate soils). This is primarily due to higher fertiliser N rate (~150 kg N/ha to sugar cane versus 50 kg N/ha to cereals), and much wetter conditions due to higher rainfalls in sugarcane regions (800mm - 4500mm) than cereal cropping regions (200mm – 700mm). Similarly, GHG emissions from horticulture and vegetable crops are similar to those from irrigated crops, primarily due to 3-7 times higher rates of fertiliser N use and irrigation than cereal crops (Table 3).
Table 3. Nitrous oxide emission factors (N2O-N/N applied in kg N/ha) for N fertiliser (NGGI 2008) Agricultural system Irrigated pasture Irrigated crops Non-irrigated pasture Non-irrigated crop Sugar cane Cotton Horticulture/vegetables
Emission factor (%) 0.4 2.1 0.4 0.3 1.25 0.5 2.1
IPCC default value or country specific (CS) CS Tier 2 CS Tier 2 CS Tier 2 IPCC Tier 1 (1%, IPCC 2006) CS Tier 2 CS Tier 2
Reducing the Carbon footprint of agriculture Broadly, there are 3 ways to reduce the carbon footprint of agriculture: (i) mitigation of GHG emissions, (ii) increasing C sequestration, and (iii) a combination of mitigation and increasing C sequestration. We will discuss these 3 approaches for the livestock industry (CH4-emissions dominant) and the grains industry (N2O emissions) of the agriculture sector. However, general principles should be applicable to other industries as well. Livestock industries Since livestock (mainly cattle and sheep) dominate in CH4 emissions through enteric fermentation, 59.3 Mt CO2-e of the total CH4 emissions from agriculture of 69.8 Mt CO2-e (Table 2). Considerable research efforts have been devoted to the management practices that can potentially reduce CH4 emissions from each animal. These include: • Changing the methanogen population of the rumen to reduce CH4 production • Reducing the livestock number • Improving livestock productivity through - Improved grazing and feeding practices (pasture type, grazing management, intensity, productivity, species, fire frequency, etc) - Specific agents and dietary additives - animal breeding • Manure management: daily spread or extensive grazing lands reduce CH4 as well as N2O emissions to 700mm in subtropical Australia). This is also supported by the recent GHG estimates from the sugar cane soils (Weijin Wang, personal communication).
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Table 4. Cumulative GHG emissions over 33 years in conventional till versus no-till cereal cropping system in southern subtropical Queensland (Wang and Dalal 2005) (values in small numerals are not significantly different from zero and therefore not added to the total emissions; negative value indicates sink)
Source
Diesel Urea-CO2 SB-CH4 SB-N2O SR-N2O Urea-N2O Sub-total Total SOC Stubble-C
Conventional till No-till Stubble burned Stubble retained Stubble burned Stubble retained N0 N90 N0 N90 N0 N90 N0 N90 Pre-farm GHG emissions (CO2-e t/ha) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 1.5 1.5 1.5 1.5 0.6 0.6 0.6 0.6 8.6 8.6 8.6 8.6 2.0 2.0 2.0 2.0 1.9 10.5 1.9 10.5 2.8 11.4 2.8 11.4 On-farm GHG emissions (CO2-e t/ha) 4.0 4.0 4.0 4.0 1.8 1.8 1.8 1.8 3.0 3.0 3.0 3.0 6.6 7.1 6.8 7.3 2.1 2.8 2.1 2.9 0.4 0.6 3.9 5.7 0.4 0.6 3.9 6.0 8.9 8.9 8.9 8.9 13.1 26.4 7.9 21.6 11.1 24.5 5.7 19.7 15.0 36.9 9.8 32.1 13.9 35.9 8.5 31.1 -3.9 1.2 -0.4 2.9 -4.5 1.8 0 -8.5 0.3 -0.05 0.2 0 -0.5 -0.6 -1.7 -0.7
Net GHG
15.0
Diesel Machinery Urea prod Herbicide Sub-total
36.9
9.3
31.5
13.9
35.9
6.8
21.9
Reducing C footprint from cropping soils involves primarily N management, option (i) but also C sequestration option (ii), since the latter also enhances soil fertility and soil health. Dalal et al. (2003) discussed the management options available for reducing N2O emissions from fertiliser N use in cropping soils. These are: Apply fertilizer N at optimum rates by taking into account all N sources available to the crop/pasture from soil (ammonium and nitrate N in the soil at the time of crop sowing, and in-crop N mineralisation), and other N sources such as manure or waste. Apply fertilizer N at the rate and time to meet crop/pasture needs and development stage, and when appropriate through split application. Avoid fertilizer N application outside the crop/pasture growing season, and especially prior to a clean fallow period. Avoid fallow periods if season or availability of irrigation permits. Provide fertilizer N application guide through crop/pasture monitoring and soil tests, and adjust fertilizer application rates and timing accordingly. Apply other nutrients if required so that nutrients supply to crop/pasture is balanced and N utilisation is optimised. Avoid surface application so that fertilizer N losses are minimised and plant utilisation maximised. Incorporate fertilizer N with soil; apply band placement or point placement close to the plant roots. Monitor and adjust fertilizer application equipment to ensure the precision and amount of fertilizer applied, and control over appropriate spatial distribution (Global Positioning System/Geographical Information System) according to the information from yield monitors, crop/pasture monitors (including remote sensing), and soil tests. Fertilizers should be in a form (such as granulated) that can be applied evenly, conveniently and costeffectively. In irrigated agricultural systems, application in sprinkler/drip irrigation may be an effective option. Fertilizer may be formulated with urease and/or nitrification inhibitors or physical coatings to synchronise fertilizer N release to that of crop/pasture growth needs so that at any given time minimum amount of mineral N (ammonium and nitrate) is present in soil. Practice good crop/pasture management, disease control and good soil management to optimise crop/pasture growth and hence efficient fertilizer N utilisation. Avoid/or reduce cultivation early in the fallow period and retain plant residues to minimise mineralisation and nitrate accumulation during the fallow period.
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Use cover crops to utilise the residual mineral N following N-fertilised main crops or mineral N accumulation following legume-leys. Also, legume-N reduces GHG by reducing fossil fuel and energy use and transport for synthetic fertiliser production.
Potential C sequestration practices • From conventional till to reduced till and no-till. Australian data suggests that in most cropping regions there is similar C input to soil under no-till and conventional till, hence limited or no additional C sequestration (Dalal and Chan 2001). It appears, however, in higher rainfall areas (>550 mm in southern Australia and >700 mm in subtropical Queensland), there may be a potential to increase C sequestration in soil under no-till. • From continuous cropping to ley pastures. Ley pastures, especially grass-legume pasture increases soil C, around 0.5 t C/ha/year or higher, during the pasture phase, but during the cropping phase soil C decreases again. However, the soil C values may still be higher than from continuous cropping. Moreover, there may also be reduction in total GHG emissions from replacement of synthetic N fertiliser (savings from manufacture, transport and CO2 release from urea hydrolysis). • From cropping to permanent pasture (for generic response, see Fig. 4). We have found that land use change from cropping to permanent pasture increases soil C for up to 35 years and may eventually attain C values similar to the soil under native vegetation or even higher if nutrient limitation is also removed (southern Australian experience). • From cropping to afforestation (for generic response, see Fig. 4) increases soil C but economic analysis is not available. Aboveground biomass may be a useful economic product. Also, Emissions Trading Scheme may provide additional incentives. • From ley pasture to permanent pasture. Usually it results in increased soil C sequestration. • From pasture to afforestation (Table 5 from Southern Queensland; for generic response, see Fig. 4). As above. Also, Emissions Trading Scheme may provide additional incentives. • From plantations to reforestation. Participating in Emissions Trading Scheme may provide additional incentives. • Use of manures and organic sources. Since manures are high in lignin, some manure C is sequestered in a slow pool of soil C and thus resides in soil longer than the labile crop residue C.
Fig. 4. Soil carbon response to various land use changes (95% confidence intervals are shown as numbers of observations are in parentheses) (adapted from Guo and Gifford, 2002).
7
Table 5. Mean total organic C stocks (t/ha) down the soil profile under different land uses at Taabinga, South Burnett, Queensland (calculated from Mathers et al. 2006). Depth (cm)
•
•
Crop (55 years)
Pasture (22 years)
Pasture22 + 4yr plantation
Native Vine Scrub
0-10
16.2
28.8
32.0
44.6
0-30
39.9
69.6
83.3
85.0
0-110
78.2
154.9
178.3
218.4
Biochar or lignite application to soil. Biochar C persists in soil much longer than the crop residue C, and it may have a place in sandy soils to increase CEC and retain fertiliser nutrients but the net GHG benefits requires Life Cycle Analysis. Lignite application has been suggested to ameliorate acidic soils in WA. It will also increase C in soil but the long-term impact of lignite application to soil is not known. Biofuel (ethanol or biodiesel) production from sugar, grain and oilseed crops. Life Cycle analysis shows that net GHG benefits may occur in biofuel production only from sugar produced from sugar cane but not from grain or oilseed crops (Crutzen et al. 2007). However, ethanol production from cellulosic materials and biodiesel from some perennial crops/plantations shows some potential as long as food and fibre production is not adversely affected and synthetic fertiliser N use is none or low.
References Crutzen, PJ, Mosier AR, Smith KA, Winiwarter W (2007) N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. Atmospheric Chemistry and Physics Discussions 7, 11191-11205. Dalal RC, Allen DE, Livesley SJ, Richards G (2008) Magnitude and biophysical regulators of methane emission and consumption in the Australian agricultural, forest, and submerged landscapes: a review. Plant and Soil 309, 43-76. Dalal RC, Chan KY (2001) Soil organic matter in rainfed cropping systems of the Australian Cereal Belt. Australian Journal of Soil Research 39, 435-464. Dalal RC, Mayer RJ (1986) Long-term trends in fertility of soils under continuous cultivation and cereal cropping in Southern Queensland. II. Total organic carbon and its rate of loss from the soil profile. Australian Journal of Soil Research 24: 281-292. Dalal RC, Wang W, Robertson GP, Parton WJ (2003) Nitrous oxide emission from Australian agricultural lands and mitigation options: a review. Australian Journal of Soil Research 41, 165-195. Guo LB, Gifford RM (2002) Soil carbon stocks and land use change: a meta analysis. Global Change Biology 8, 345-360. IPCC (2007) Climate Change 2007: The Physical Science Basis. Cambridge University Press, Cambridge, UK. Mathers N, Dalal RC, Moody PW (2006) Mareseni (2006) Carbon sequestration: a case study from the South Burnett. In: Jones, C.E. and Clarke, K. (eds) Proceedings ‘Managing the Carbon Cycle’, Kingaroy Forum, 25-26 Oct. 2006. Publisher Carbon For Life Inc. pp. 31-34. NGGI (2008) National Greenhouse Gas Inventory 2006. Department of Climate Change, Commonwealth of Australia. Reid RS, Thornton PK, McCrabb GJ, Kruska RL, Atieno F, Jones PG (2004) Is it possible to mitigate greenhouse gas emissions in pastoral ecosystems of the tropics? Environment, Development and Sustainability 6, 91109. Smith P, Martino D, Cai Z, Gwary D, Janzen H, Kumar P, McCarl B, Ogle S, O’Mara F, Rice C, Scholes R, Sirotenko O, Howden M, McAllister T, Pan G, Romanenkov V, Schneider U, Towprayoon S, Wattenbach M, Smith J (2008) Greenhouse gas mitigation in agriculture. Philosophical Transactions of the Royal Society, 363B, 789-813. Wang WJ, Dalal RC (2006) Carbon inventory for a cereal cropping system under contrast tillage, nitrogen fertilisation and stubble management practices. Soil and Tillage Research 91, 68-74. Watson RT, Noble IR, Bolin B, Ravindranath NH, Verardo DJ, Dokken DJ (2000) Land Use, Land-use Change, and Forestry: A Special Report of the IPCC. 337pp, Cambridge University Press: Cambridge, UK.
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Queensland Department of Natural Resources and Water School of Land, Crop and Food Sciences, University of Queensland, St Lucia
Introduction Agriculture has always been about farming carbon. Until the beginning of industrial revolution, carbon outputs were in close synchrony with carbon inputs (plant biomass including vegetation fallows, manures, etc), and relatively small land area used for growing food and fibre crops. Carbon dioxide levels in the atmosphere before 1750 have been maintained between 180 ppm and 280ppm for at least 10,000 years and 380ppm, CH4 from 715ppb to 1774ppb and N2O from 270ppb to 320ppb by 2005 (IPCC 2007). Agriculture contributes about 16%, 40% and 36% of both natural and anthropogenic global emissions of CO2, CH4 and N2O emissions, so-called the carbon footprint of agriculture. We present the carbon footprint of Australian agriculture and management options to increase C sequestration and reduce N2O and CH4 emissions from agriculture. The Carbon cycle and major Greenhouse Gases (CO2, CH4 and N2O) Natural processes such as photosynthesis, weathering by silicate rocks and carbonate formation, and carbon fixed by marine plants and consequent burial of marine sediments provide the major carbon sinks. Respiration, decay, acidification of carbonates and sea-surface gas exchange are the major natural processes of carbon source. Fossil fuel burning, deforestation or land clearing followed by biomass burning are the major human-induced sources of CO2 into the atmosphere. CH4 is produced primarily by methanogens (Archea domain) by the process of methanogenesis under anaerobic conditions. On the other hand, CH4 oxidisers (methanotrophs) and consumers (sinks) are aerobic microbes in soil, requiring O2 for CH4 oxidation. N2O is produced in soil by at least three microbial-mediated mechanisms: (i) nitrification, utilising nitrite as an alternative electron acceptor, thereby, reducing it to N2O (via NO), (ii) dissimilatory nitrate reduction (denitrification) and (iii) assimilatory nitrate reduction. Over the last 250 years, coinciding with the beginning of industrial revolution, CO2 levels in the atmosphere have increased from 280 ppm to 384 ppm in 2008, the highest atmospheric CO2 concentration since at least 650, 000 years ago. Concentration of CH4 in the atmosphere has increased from 715 ppb to 1775 ppb and that of N2O from 270 ppb to 320 ppb since 1750AD. IPCC (2007) summarised the global greenhouse gas fluxes of C, CH4 and N2O (also O3) for 2005, which is presented in Figure 1. It has generally been accepted that CO2 emissions and sinks (as CO2 fixed during photosynthesis by vegetation and sequestered as soil organic C and carbonate C) in natural ecosystems vary very little. In fact, natural ecosystems provide net C uptake of -2.6 Gt (1 Gt = 1015 g = billion tonne) C/year, resulting in the additional terrestrial CO2 sink of ~ -0.9 Gt C/year (net release from land use change, 1.6 Gt C/year), as compared to C sink of -2.2 Gt C/year provided by oceans (Fig. 1). Thus, the natural ecosystems absorb almost 45% of anthropogenic GHG emissions of >7.2 Gt C/year, primarily from fossil fuel use and cement production. Almost two-third of CH4 emissions are from anthropogenic activities such as energy production and use, enteric fermentation and animal wastes, paddy rice, domestic sewage and landfills, and biomass burning. Almost 6% of atmospheric CH4 is consumed by bacteria in soil (Dalal et al. 2007). Emissions of N2O from human activities comprise 45% of the total global N2O emissions; of that 80% originates from soil. For total C stocks, refer to the IPCC (2007) report. In summary, surface oceans and surface sediments comprise 1050 Gt C, intermediate and deep oceans contain 37,100 Gt C, mostly in carbonate-C. Among the terrestrial systems, soil provides the major C sink (1550 Gt as organic C and 950 Gt as carbonateC), and vegetation provides 760 Gt C sink. Atmosphere contains about 760 Gt C, similar to that contained in vegetation, although 165 Gt C of the total stock is the annual C flux between atmosphere and land and oceans. In summary, the net GHG annual rate of increases in 2005 were estimated to be 4.1± 0.1 Gt of CO2-C (annual emissions, 650 Gt C, annual sinks, 646 Gt C), 2 Mt of CH4 (annual emissions, 582 Mt CH4, annual sinks, 580 Mt CH4), and 6 Mt of N2O (annual emissions, 28 Mt N2O; annual sinks, 22 Mt N2O).
Figure 1. Global GHG flux in 2005. (a) C fluxes showing an annual increase of 4.1 Gt C/year. Heterotrophic respiration (119.6 Gt C/year) and gross terrestrial primary production (~120 Gt C/year) are considered to have neutral effect on global C flux (not shown). (b) CH4 fluxes are dominated by anthropogenic emissions. CH4 oxidation in natural soils forms a significant CH4 sink. (c) N2O emissions from anthropogenic sources comprise almost 45% of the total annual N2O fluxes to the atmosphere. (d) Annual O3 fluxes; O3 lifetime in the atmosphere is less than a month. Global warming potentials of CH4 and N2O are 25 and 298 as compared to CO2 on 100-year time scale (IPCC 2007). The Australian greenhouse gas (GHG) fluxes Greenhouse gas flux estimates for Australia are provided annually (3-year moving averages) by the National Greenhouse Gas Inventory Committee, which is submitted to UNFCCC every year. The latest GHG estimates are available for 2006 (Table 1). Compared to 1990 baseline, GHG emissions in 2006 increased by 4.2% in line with 8% increase allowed for under Kyoto Protocol for the 2008-2012 period. The increase in GHG emissions from the energy sector by 40% (in case of Queensland by 90%) were substantially countered by decrease in GHG emissions from the land use change sector by 54% (about similar percent in Queensland) to enable to achieve the agreed Kyoto Protocol target. Table 1. National greenhouse gas emissions by sector in Australia for 2006 (Queensland values for the major sectors are given in parentheses) (NGGI 2008) Major sector
1990
Industrial processes Agriculture Waste Land use change Forestry
2006 Mt CO2-e /year 286.4 (50.6) 400.9 (96.0) 195.1 287.4 62.1 79.1 29.2 34.5 24.1 28.4 86.8 (23.6) 90.1 (26.4) 18.8 16.6 136.5 (89.0) 62.9 (41.7) 0 -23.0
Change (%) 40.0 (89.7) 47.3 27.4 18.1 17.7 3.8 (11.9) -11.4 -53.9 (-52.6)
Total net emissions
552.6 (169.8)
4.2 (0.7)
Energy Stationary energy Transport Fugitive emissions
2
576.0 (170.9)
Agriculture sector Total GHG emissions from agriculture sector were an estimated 90.1 Mt CO2-e emissions or 16% of net national emissions in 2006 (Table 2). Methane emissions from livestock and N2O emissions from agricultural soils, mainly from N fertiliser application were the dominant sources, accounting for 58.0% and 80.7%, respectively, of the net national emissions for these two gases. GHG emissions from livestock, including the enteric fermentation and manure management, declined by 4.7% (3.1 Mt) between 1990 and 2006, due to decrease in sheep numbers by 47%, which was partly offset by 14% increase in cattle numbers. There was a 30.6% (6.4 Mt) increase in emissions from the remaining agricultural activities between 1990 and 2006, mainly from increasing N fertiliser use and burning of savannas. However, there has been a small increase (3.8%) in overall GHG emissions from the agriculture sector between 1990 and 2006.
Table 2. Agriculture sector greenhouse gas emissions in 2006 (NGGI 2008) (emissions from enteric fermentation and agric. soils for Qld in parentheses) Category
CO2
Enteric fermentation
na
Manure management Rice cultivation Agricultural soils
na na na
Prescribed burning of savannas Field burning of agric. residues Total
na na na
Solid waste disposal on land
CH4 N 2O Mt CO2-e/year 59.3 na (21.9) 2.0 1.6 0.3 na na 15.2 (4.5) 8.1 3.4 0.2 0.1 69.8 20.3 13.2
0.6
Total 59.3
Category (%) 65.8
3.6 0.3 15.2
4.0 0.3 16.9
11.5 0.3 90.1 (26.4) 13.2
12.8 0.3 100
Total GHG (%) 10.3 (12.8) 0.6 0.05 2.6 (2.6) 2.0 0.1 15.6 (15.5) 2.3
In summary, there are three major categories of GHG emissions from the agriculture sector: CH4 emissions from livestock, N2O emissions from N fertiliser use, and CH4 and N2O emissions from savanna burning. For Queensland, the first two sources dominate GHG emissions from agriculture.
cotton sugarcane
Horticulture / vegetables
Irrigated crop Non- irrigated pasture
Irrigated pasture Non- irrigated crop
Figure 2. Direct GHG emissions from cropping soils and biomass burning were 27 Mt CO2-e in 2006, primarily from fertiliser N use and N mineralisation (NGGI 2008) Although sugarcane uses only 10% of total annual N fertiliser applied to cropping soils compared to 70% used for cereal crops, net GHG emissions are similar or even higher than those from cereal crops (Fig.
3
2). Currently, IPCC default value of 1.25% of fertiliser N for sugarcane is used in NGGI (Table 3). Field estimates of N2O emission factor varies from 1% (non-acid sulphate soils) to 10% (acid sulphate soils). This is primarily due to higher fertiliser N rate (~150 kg N/ha to sugar cane versus 50 kg N/ha to cereals), and much wetter conditions due to higher rainfalls in sugarcane regions (800mm - 4500mm) than cereal cropping regions (200mm – 700mm). Similarly, GHG emissions from horticulture and vegetable crops are similar to those from irrigated crops, primarily due to 3-7 times higher rates of fertiliser N use and irrigation than cereal crops (Table 3).
Table 3. Nitrous oxide emission factors (N2O-N/N applied in kg N/ha) for N fertiliser (NGGI 2008) Agricultural system Irrigated pasture Irrigated crops Non-irrigated pasture Non-irrigated crop Sugar cane Cotton Horticulture/vegetables
Emission factor (%) 0.4 2.1 0.4 0.3 1.25 0.5 2.1
IPCC default value or country specific (CS) CS Tier 2 CS Tier 2 CS Tier 2 IPCC Tier 1 (1%, IPCC 2006) CS Tier 2 CS Tier 2
Reducing the Carbon footprint of agriculture Broadly, there are 3 ways to reduce the carbon footprint of agriculture: (i) mitigation of GHG emissions, (ii) increasing C sequestration, and (iii) a combination of mitigation and increasing C sequestration. We will discuss these 3 approaches for the livestock industry (CH4-emissions dominant) and the grains industry (N2O emissions) of the agriculture sector. However, general principles should be applicable to other industries as well. Livestock industries Since livestock (mainly cattle and sheep) dominate in CH4 emissions through enteric fermentation, 59.3 Mt CO2-e of the total CH4 emissions from agriculture of 69.8 Mt CO2-e (Table 2). Considerable research efforts have been devoted to the management practices that can potentially reduce CH4 emissions from each animal. These include: • Changing the methanogen population of the rumen to reduce CH4 production • Reducing the livestock number • Improving livestock productivity through - Improved grazing and feeding practices (pasture type, grazing management, intensity, productivity, species, fire frequency, etc) - Specific agents and dietary additives - animal breeding • Manure management: daily spread or extensive grazing lands reduce CH4 as well as N2O emissions to 700mm in subtropical Australia). This is also supported by the recent GHG estimates from the sugar cane soils (Weijin Wang, personal communication).
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Table 4. Cumulative GHG emissions over 33 years in conventional till versus no-till cereal cropping system in southern subtropical Queensland (Wang and Dalal 2005) (values in small numerals are not significantly different from zero and therefore not added to the total emissions; negative value indicates sink)
Source
Diesel Urea-CO2 SB-CH4 SB-N2O SR-N2O Urea-N2O Sub-total Total SOC Stubble-C
Conventional till No-till Stubble burned Stubble retained Stubble burned Stubble retained N0 N90 N0 N90 N0 N90 N0 N90 Pre-farm GHG emissions (CO2-e t/ha) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 1.5 1.5 1.5 1.5 0.6 0.6 0.6 0.6 8.6 8.6 8.6 8.6 2.0 2.0 2.0 2.0 1.9 10.5 1.9 10.5 2.8 11.4 2.8 11.4 On-farm GHG emissions (CO2-e t/ha) 4.0 4.0 4.0 4.0 1.8 1.8 1.8 1.8 3.0 3.0 3.0 3.0 6.6 7.1 6.8 7.3 2.1 2.8 2.1 2.9 0.4 0.6 3.9 5.7 0.4 0.6 3.9 6.0 8.9 8.9 8.9 8.9 13.1 26.4 7.9 21.6 11.1 24.5 5.7 19.7 15.0 36.9 9.8 32.1 13.9 35.9 8.5 31.1 -3.9 1.2 -0.4 2.9 -4.5 1.8 0 -8.5 0.3 -0.05 0.2 0 -0.5 -0.6 -1.7 -0.7
Net GHG
15.0
Diesel Machinery Urea prod Herbicide Sub-total
36.9
9.3
31.5
13.9
35.9
6.8
21.9
Reducing C footprint from cropping soils involves primarily N management, option (i) but also C sequestration option (ii), since the latter also enhances soil fertility and soil health. Dalal et al. (2003) discussed the management options available for reducing N2O emissions from fertiliser N use in cropping soils. These are: Apply fertilizer N at optimum rates by taking into account all N sources available to the crop/pasture from soil (ammonium and nitrate N in the soil at the time of crop sowing, and in-crop N mineralisation), and other N sources such as manure or waste. Apply fertilizer N at the rate and time to meet crop/pasture needs and development stage, and when appropriate through split application. Avoid fertilizer N application outside the crop/pasture growing season, and especially prior to a clean fallow period. Avoid fallow periods if season or availability of irrigation permits. Provide fertilizer N application guide through crop/pasture monitoring and soil tests, and adjust fertilizer application rates and timing accordingly. Apply other nutrients if required so that nutrients supply to crop/pasture is balanced and N utilisation is optimised. Avoid surface application so that fertilizer N losses are minimised and plant utilisation maximised. Incorporate fertilizer N with soil; apply band placement or point placement close to the plant roots. Monitor and adjust fertilizer application equipment to ensure the precision and amount of fertilizer applied, and control over appropriate spatial distribution (Global Positioning System/Geographical Information System) according to the information from yield monitors, crop/pasture monitors (including remote sensing), and soil tests. Fertilizers should be in a form (such as granulated) that can be applied evenly, conveniently and costeffectively. In irrigated agricultural systems, application in sprinkler/drip irrigation may be an effective option. Fertilizer may be formulated with urease and/or nitrification inhibitors or physical coatings to synchronise fertilizer N release to that of crop/pasture growth needs so that at any given time minimum amount of mineral N (ammonium and nitrate) is present in soil. Practice good crop/pasture management, disease control and good soil management to optimise crop/pasture growth and hence efficient fertilizer N utilisation. Avoid/or reduce cultivation early in the fallow period and retain plant residues to minimise mineralisation and nitrate accumulation during the fallow period.
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Use cover crops to utilise the residual mineral N following N-fertilised main crops or mineral N accumulation following legume-leys. Also, legume-N reduces GHG by reducing fossil fuel and energy use and transport for synthetic fertiliser production.
Potential C sequestration practices • From conventional till to reduced till and no-till. Australian data suggests that in most cropping regions there is similar C input to soil under no-till and conventional till, hence limited or no additional C sequestration (Dalal and Chan 2001). It appears, however, in higher rainfall areas (>550 mm in southern Australia and >700 mm in subtropical Queensland), there may be a potential to increase C sequestration in soil under no-till. • From continuous cropping to ley pastures. Ley pastures, especially grass-legume pasture increases soil C, around 0.5 t C/ha/year or higher, during the pasture phase, but during the cropping phase soil C decreases again. However, the soil C values may still be higher than from continuous cropping. Moreover, there may also be reduction in total GHG emissions from replacement of synthetic N fertiliser (savings from manufacture, transport and CO2 release from urea hydrolysis). • From cropping to permanent pasture (for generic response, see Fig. 4). We have found that land use change from cropping to permanent pasture increases soil C for up to 35 years and may eventually attain C values similar to the soil under native vegetation or even higher if nutrient limitation is also removed (southern Australian experience). • From cropping to afforestation (for generic response, see Fig. 4) increases soil C but economic analysis is not available. Aboveground biomass may be a useful economic product. Also, Emissions Trading Scheme may provide additional incentives. • From ley pasture to permanent pasture. Usually it results in increased soil C sequestration. • From pasture to afforestation (Table 5 from Southern Queensland; for generic response, see Fig. 4). As above. Also, Emissions Trading Scheme may provide additional incentives. • From plantations to reforestation. Participating in Emissions Trading Scheme may provide additional incentives. • Use of manures and organic sources. Since manures are high in lignin, some manure C is sequestered in a slow pool of soil C and thus resides in soil longer than the labile crop residue C.
Fig. 4. Soil carbon response to various land use changes (95% confidence intervals are shown as numbers of observations are in parentheses) (adapted from Guo and Gifford, 2002).
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Table 5. Mean total organic C stocks (t/ha) down the soil profile under different land uses at Taabinga, South Burnett, Queensland (calculated from Mathers et al. 2006). Depth (cm)
•
•
Crop (55 years)
Pasture (22 years)
Pasture22 + 4yr plantation
Native Vine Scrub
0-10
16.2
28.8
32.0
44.6
0-30
39.9
69.6
83.3
85.0
0-110
78.2
154.9
178.3
218.4
Biochar or lignite application to soil. Biochar C persists in soil much longer than the crop residue C, and it may have a place in sandy soils to increase CEC and retain fertiliser nutrients but the net GHG benefits requires Life Cycle Analysis. Lignite application has been suggested to ameliorate acidic soils in WA. It will also increase C in soil but the long-term impact of lignite application to soil is not known. Biofuel (ethanol or biodiesel) production from sugar, grain and oilseed crops. Life Cycle analysis shows that net GHG benefits may occur in biofuel production only from sugar produced from sugar cane but not from grain or oilseed crops (Crutzen et al. 2007). However, ethanol production from cellulosic materials and biodiesel from some perennial crops/plantations shows some potential as long as food and fibre production is not adversely affected and synthetic fertiliser N use is none or low.
References Crutzen, PJ, Mosier AR, Smith KA, Winiwarter W (2007) N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. Atmospheric Chemistry and Physics Discussions 7, 11191-11205. Dalal RC, Allen DE, Livesley SJ, Richards G (2008) Magnitude and biophysical regulators of methane emission and consumption in the Australian agricultural, forest, and submerged landscapes: a review. Plant and Soil 309, 43-76. Dalal RC, Chan KY (2001) Soil organic matter in rainfed cropping systems of the Australian Cereal Belt. Australian Journal of Soil Research 39, 435-464. Dalal RC, Mayer RJ (1986) Long-term trends in fertility of soils under continuous cultivation and cereal cropping in Southern Queensland. II. Total organic carbon and its rate of loss from the soil profile. Australian Journal of Soil Research 24: 281-292. Dalal RC, Wang W, Robertson GP, Parton WJ (2003) Nitrous oxide emission from Australian agricultural lands and mitigation options: a review. Australian Journal of Soil Research 41, 165-195. Guo LB, Gifford RM (2002) Soil carbon stocks and land use change: a meta analysis. Global Change Biology 8, 345-360. IPCC (2007) Climate Change 2007: The Physical Science Basis. Cambridge University Press, Cambridge, UK. Mathers N, Dalal RC, Moody PW (2006) Mareseni (2006) Carbon sequestration: a case study from the South Burnett. In: Jones, C.E. and Clarke, K. (eds) Proceedings ‘Managing the Carbon Cycle’, Kingaroy Forum, 25-26 Oct. 2006. Publisher Carbon For Life Inc. pp. 31-34. NGGI (2008) National Greenhouse Gas Inventory 2006. Department of Climate Change, Commonwealth of Australia. Reid RS, Thornton PK, McCrabb GJ, Kruska RL, Atieno F, Jones PG (2004) Is it possible to mitigate greenhouse gas emissions in pastoral ecosystems of the tropics? Environment, Development and Sustainability 6, 91109. Smith P, Martino D, Cai Z, Gwary D, Janzen H, Kumar P, McCarl B, Ogle S, O’Mara F, Rice C, Scholes R, Sirotenko O, Howden M, McAllister T, Pan G, Romanenkov V, Schneider U, Towprayoon S, Wattenbach M, Smith J (2008) Greenhouse gas mitigation in agriculture. Philosophical Transactions of the Royal Society, 363B, 789-813. Wang WJ, Dalal RC (2006) Carbon inventory for a cereal cropping system under contrast tillage, nitrogen fertilisation and stubble management practices. Soil and Tillage Research 91, 68-74. Watson RT, Noble IR, Bolin B, Ravindranath NH, Verardo DJ, Dokken DJ (2000) Land Use, Land-use Change, and Forestry: A Special Report of the IPCC. 337pp, Cambridge University Press: Cambridge, UK.
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